ELSEVIER Catalysis Today 29 (1996) 99-102

CATALYSIS TODAY

NO x reduction with methane over mordenite supported palladium catalyst

Hiroshi Uchida *, Ken-ichi Yamaseki, Iruru Takahashi Fundamental Technology Research Laboratory, Tokyo Gas Co., Ltd., 16-25, Shibaura, 1-Chome, Minato-ku, Tokyo, Japan

Abstract

Catalytic reduction of NO x with small amounts of hydrocarbons in the presence of excess oxygen and water vapor have been studied over mordenite supported metal catalysts. Pd /morden i te catalyst was found to be very active for the reduction

of NO x with methane.

Keywords: NO x reduction; Palladium

1. Introduction

Pollution caused by the emission of NO x is one of the most important global environmental problems today.

Several methods of removing NO x from ex- haust gases, including selective catalytic reduc- tion with ammonia (SCR) and catalytic reduc- tion using three-way catalysts, have been adopted. These conventional methods cannot, however, be readily applied to lean-burn gas engines in cogeneration systems because ammo- nia is difficult to deal with due to its poisonous character and three-way catalysts are effective only in the absence of oxygen. Recently, al- though many researchers have been studying catalytic reduction of NO x with hydrocarbons [1-5], favorable catalytic systems for gas en- gines have not yet been developed. In view of this, we have been studying and developing new

* Corresponding author.

catalysts for the reduction of NO x with oxy- genated compounds and hydrocarbons in an at- tempt to find a better way to remove NO x from the exhaust gases of lean-burn gas engines in cogeneration systems [6].

This paper presents the results of studies of catalytic reduction of NO x with methane over mordenite supported metal catalysts in the pres- ence of excess oxygen and water vapor.

2. Experimental

Catalysts were prepared by an ion-exchange procedure. Catalytic activity measurements were carried out in a fixed-bed flow reactor under ambient pressure. Space velocity was 44000 h -1. The reaction mixture typically contained 91-180 ppm of NO, 910 ppm of CO, 6.4% of CO 2, 9.1% of 02, 9.1% of H20, 2450 ppm of hydrocarbon (C 1 basis), diluted with N 2. The effluent gases were analyzed by a chemilumi-

0920-5861/96/$32.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 9 2 0 - 5 8 6 1 ( 9 5 ) 0 0 2 8 6 - 3

i00 H. Uchida et al. / Catalysis Today 29 (1996) 99-102

1 0 0 , • , -

- o "i 60

~ 4o

~ 2o Z

o 2 4 6 8 10 Water vapor concentrat ion (%)

Fig. 1. Effect of water vapor. Catalyst: Pd/mordenite, Tempera- ture: 673 K, [NO]=200 ppm, [CO]= 1000 ppm, [CO2]=7%, [02 ] = 10%, [CH 4 ] = 2700 ppm (dry basis).

8O

v

o~ 4o

201

NOx O

CH 4

Oxygen concentrat ion (%)

Fig. 2. Effect of oxygen. Catalyst: Pd/mordenite, Temperature: 673 K, [NO] = 91 ppm, [CO] = 910 ppm, [CO2] = 6.4%, [H20] = 9.1%, [CH4]= 2450 ppm.

nescence NO x analyzer and a FID gas chro- matograph. The NO~ conversion was calculated based on the differing NO x concentrations at the inlet and outlet of the reactor.

3. Results and discussion

In the presence of excess oxygen, NO x is reduced with hydrocarbons and oxygen over the catalyst, and the oxidation of hydrocarbons oc- curs simultaneously.

The effect of water vapor on NO/ reduction with methane over Pd/mordenite catalyst in the presence of excess oxygen is shown in Fig. 1. At 673 K, NO~ conversion in the absence of water vapor reaches 95%. NO~ conversion de- creases with increasing the concentration of wa- ter vapor. The effect of water vapor is com- pletely reversible. This suggests that the com- petitive adsorption of water and NO x for active Pd sites depresses NO x reduction with methane.

The effect of oxygen on NO x reduction with methane over Pd/mordenite catalyst in the presence of water vapor is shown in Fig. 2. In the absence of oxygen, the Pd/mordeni te cata- lyst is very active for NO~ reduction with methane, and in the presence of oxygen, NOx conversion declines. However, although NO x conversion is low at the low concentration of oxygen, when the concentration of oxygen is increased, NO x conversion rises. The introduc-

tion of oxygen inhibits and then enhances NO x reduction with methane over Pd/mordenite cat- alyst.

The results of NO x reduction with methane over mordenite supported metal catalysts in the presence of excess oxygen and water vapor are shown in Fig. 3. NO is converted to N 2 and N O 2 with methane and oxygen over Co/mordenite catalyst, Cu/mordenite catalyst, Rh/mordenite catalyst and Pt/mordenite cata- lyst. However, over the Pd/mordenite catalyst, NO is almost completely converted to N 2. In the presence of excess oxygen and water vapor, the order of NO~ conversion is Pd/mordenite cata- lyst > Pt /mordeni te catalyst > Rh/mordenite c a t a l y s t > C u / m o r d e n i t e c a t a l y s t >

Pt

Pd

0 20 4o 60 80 100 N O x convers ion (%)

Fig. 3. Effect of supported metals on mordenite. Temperature: 673 K, [NO] = 180 ppm, [CO] = 910 pprn, [CO 2 ] ~ 6.4%, [ 0 2 ] = 9.1%, [H20]= 9.1%, [CH 4 ] = 2450 ppm.

H. Uchida et al. / Catalysis Today 29 (1996) 99-102 101

CH4

c . ~

CsH8

C4Hlo

' 2 o ' ' ' ' ' o ' 40 60 8 100 N O x conversion (%)

Fig. 4. Effect of reducing agents. Catalyst: Pd/mordenite, Tem- perature: 673 K, [NO] = 180 ppm, [CO] = 910 ppm, [CO2 ] = 6.4%, [02 ] = 9.1%, [H20] = 9.1%, [Hydrocarbon] = 2450 ppm (C I ba- sis).

m 100 i i /m i

~ N g 60

o~ ~ ~ , ! C H 4 i ~ 40 / / /CH4

20

0 , I 550 600 650 700 750

Temperature (K)

Fig. 5. Effect of reaction temperature. Catalyst: Pd/mordenite, [NO] = 180 ppm, [CO] = 910 ppm, [CO 2 ] = 6.4%, [0 2 ] = 9.1%, [H20] = 9.1%, [CH 4 ] = 2450 ppm.

Co/mordeni te catalyst. The Pd/mordeni te cata- lyst shows an extremely high activity and selec- tivity for NO x reduction with methane. This suggests that methane is readily activated over Pd/mordeni te catalyst in the presence of excess oxygen and water vapor.

Various hydrocarbons including methane, ethane, propane, and butane were investigated as NO x reducing agents. Fig. 4 shows the re- sults of NO x reduction with various hydrocar- bons over Pd/mordenite catalyst in the pres- ence of excess oxygen and water vapor. Over Pd/mordeni te catalyst, methane, which is the principal ingredient of fuel for gas engines, is the most effective reducing agent among satu- rated hydrocarbons in the presence of excess oxygen and water vapor. Ethane, propane and butane show lower effectiveness in NO x reduc- tion than methane because they are readily oxi- dized by oxygen.

Over Co/mordenite catalyst, propane acts effectively as a reducing agent comparable to that of methane in the presence of excess oxy- gen and water vapor, although it is lower.

Fig. 5 shows the results of NOx reduction with methane over Pd/mordeni te catalyst as a function of reaction temperature. NO x conver- sion increases as the temperature rises to 673 K, with the maximum NO x conversion reaching 80%. However, NO~ conversion falls when the

temperature exceeds 673 K. The oxidation of methane by oxygen dominates the NO x reduc- tion at temperatures over 673 K. The methane conversion increases with reaction temperature. Because the methane oxidation rate becomes significant at high temperatures, the depressed NO x conversion at temperature above 673 K is most likely the result of lowered methane con- centrations in the catalyst bed.

NO x reduction is dependent on methane con- centration. As shown in Fig. 6, NO x conversion increases with methane concentration. NO x re- duction rate is proportional to methane concen- tration. On the other hand, methane conversion decreases with increasing methane concentra- tion.

100 100

9O

• ~ 80

7o o

o ~ 60 Z

50 0

' I I I

, I i I i I

2000 4000 6000 CH 4 concentration (ppm)

8o

60 .~

4o ~ 8

20 f f

' 0 8000

Fig. 6. Effect of methane concentration. Catalyst: Pd/mordenite, Temperature: 673 K, [NO]= 180 ppm, [CO] = 910 ppm, [CO2]= 6.4%, [02]= 9.1%, [H20]= 9.1%, [CH4]= 1350-6360 ppm.

102 H. Uchida et al. / Catalysis Today 29 (1996) 99-102

4. Conclusion References

We report that the Pd/mordenite catalyst is very active for the reduction of NO x with methane in the presence of excess oxygen and vapor water. This conclusion provides an alter- native approach for the emission control of lean-burn gas engines in cogeneration systems.

[1] Y. Li and J.N. Armor, Appl. Catai. B, 2 (1993) 239. [2] M. Iwamoto and N. Mizuno, J. Automobile Eng., 207 (1993)

23. [3] Y. Li and J.N. Armor, Appl. Catal. B, ! (1992) L31. [4] H. Hamada, Y. Kintaichi, M. Sasaki and T. Ito, Appi. Catal.

A, 88 (1992) L1. [5] M. lwamoto and H. Hamada, Catal. Today, 10 (1991) 57. [6] H. Uchida, K. Yamaseki and I. Takahashi, Proc. 1st World

Conf. Environ. Catal., (1995) 331.